U.S. Secretary of State John Kerry on May 13 has warned Moscow that now is a critical time to fulfil a shaky cease-fire deal in eastern Ukraine, after holding lengthy talks with President Vladimir Putin.

Kerry told a meeting of NATO foreign ministers in Turkey that now is a "critical moment for action by Russia and separatists to live up to the Minsk agreement" signed in the Belarusian capital earlier this year.

The two-day NATO gathering on May 13 is due to discuss the continued crisis in Ukraine and instability throughout the Middle East, including Syria and Iraq.

In opening remarks, Kerry said the time has come for separatists in eastern Ukraine to live up to the cease-fire agreement signed in Minsk in February.

Kerry, who spent all day on May 12 in Russia's Black Sea resort of Sochi, where he and Putin discussed Ukraine, said this was a "critical moment" for Russia and the separatists to fulfil the truce deal.

"This is an enormous moment of opportunity for the conflict there to find a path to certainty and resolution," Kerry said.